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Given the different numbers of “letters” in the mRNA and protein “alphabets,” combinations of nucleotides corresponded to single amino acids. Using a three-nucleotide code means that there are a total of 64 (4 × 4 × 4) possible combinations; therefore, a given amino acid is encoded by more than one nucleotide triplet ( [link] ).

Figure shows all 64 codons. Sixty-two of these code for amino acids, and three are stop codons shown in red. The start codon, AUG, is colored green.
This figure shows the genetic code for translating each nucleotide triplet, or codon, in mRNA into an amino acid or a termination signal in a nascent protein. (credit: modification of work by NIH)

Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets are called stop codons . Another codon, AUG, also has a special function. In addition to specifying the amino acid methionine, it also serves as the start codon    to initiate translation. The reading frame for translation is set by the AUG start codon near the 5' end of the mRNA. The genetic code is universal. With a few exceptions, virtually all species use the same genetic code for protein synthesis, which is powerful evidence that all life on Earth shares a common origin.

The mechanism of protein synthesis

Just as with mRNA synthesis, protein synthesis can be divided into three phases: initiation, elongation, and termination. The process of translation is similar in prokaryotes and eukaryotes. Here we will explore how translation occurs in E. coli , a representative prokaryote, and specify any differences between prokaryotic and eukaryotic translation.

Protein synthesis begins with the formation of an initiation complex. In E. coli , this complex involves the small ribosome subunit, the mRNA template, three initiation factors, and a special initiator tRNA. The initiator tRNA interacts with the AUG start codon, and links to a special form of the amino acid methionine that is typically removed from the polypeptide after translation is complete.

In prokaryotes and eukaryotes, the basics of polypeptide elongation are the same, so we will review elongation from the perspective of E. coli . The large ribosomal subunit of E. coli consists of three compartments: the A site binds incoming charged tRNAs (tRNAs with their attached specific amino acids). The P site binds charged tRNAs carrying amino acids that have formed bonds with the growing polypeptide chain but have not yet dissociated from their corresponding tRNA. The E site releases dissociated tRNAs so they can be recharged with free amino acids. The ribosome shifts one codon at a time, catalyzing each process that occurs in the three sites. With each step, a charged tRNA enters the complex, the polypeptide becomes one amino acid longer, and an uncharged tRNA departs. The energy for each bond between amino acids is derived from GTP, a molecule similar to ATP ( [link] ). Amazingly, the E. coli translation apparatus takes only 0.05 seconds to add each amino acid, meaning that a 200-amino acid polypeptide could be translated in just 10 seconds.

Illustration shows the steps of protein synthesis. First, an initiator tRNA recognizes the sequence AUG on the mRNA that is associated with the small ribosomal subunit. The large subunit joins the complex. Next, a second tRNA is recruited at the A site. A peptide bond is formed between the first amino acid, which is at the P site, and the second amino acid, which is at the A site. The mRNA then shifts and the first tRNA is moved to the E site, where it dissociates from the ribosome. Another tRNA binds the A site, and the process is repeated.
Translation begins when a tRNA anticodon recognizes a codon on the mRNA. The large ribosomal subunit joins the small subunit, and a second tRNA is recruited. As the mRNA moves relative to the ribosome, the polypeptide chain is formed. Entry of a release factor into the A site terminates translation and the components dissociate.

Termination of translation occurs when a stop codon (UAA, UAG, or UGA) is encountered. When the ribosome encounters the stop codon, the growing polypeptide is released and the ribosome subunits dissociate and leave the mRNA. After many ribosomes have completed translation, the mRNA is degraded so the nucleotides can be reused in another transcription reaction.

Concept in action

Transcribe a gene and translate it to protein using complementary pairing and the genetic code at this site .

Section summary

The central dogma describes the flow of genetic information in the cell from genes to mRNA to proteins. Genes are used to make mRNA by the process of transcription; mRNA is used to synthesize proteins by the process of translation. The genetic code is the correspondence between the three-nucleotide mRNA codon and an amino acid. The genetic code is “translated” by the tRNA molecules, which associate a specific codon with a specific amino acid. The genetic code is degenerate because 64 triplet codons in mRNA specify only 20 amino acids and three stop codons. This means that more than one codon corresponds to an amino acid. Almost every species on the planet uses the same genetic code.

The players in translation include the mRNA template, ribosomes, tRNAs, and various enzymatic factors. The small ribosomal subunit binds to the mRNA template. Translation begins at the initiating AUG on the mRNA. The formation of bonds occurs between sequential amino acids specified by the mRNA template according to the genetic code. The ribosome accepts charged tRNAs, and as it steps along the mRNA, it catalyzes bonding between the new amino acid and the end of the growing polypeptide. The entire mRNA is translated in three-nucleotide “steps” of the ribosome. When a stop codon is encountered, a release factor binds and dissociates the components and frees the new protein.

Questions & Answers

what Is taxonom
Don Reply
Taxonom is the the laws and principles guiding the classification of living things either plant 🌿 or animal
Samuel
protein synthesis began with what?
Kesselly Reply
What structures does a plant cell have that an animal cell does not hav
Martin Reply
A plant cell has a cell wall, a large central vacuole, and chloroplasts, which an animal cell does not have.
AI-Robot
what is the function of the cytoskeleton?
Martin
truly. Al-robot
Mark
why is it advantageous for the cell membrane to be fluid in nature?
Martin Reply
The fluid nature of the cell membrane allows for flexibility and dynamic changes in its structure. This allows for the movement of molecules and ions in and out of the cell, as well as the ability to respond to external stimuli. The fluidity of the membrane also allows for the formation of specialized structures such as receptor proteins, which are important for cell-cell communication.
AI-Robot
What is used as a visual marker for successful gene transfer into transgenic animals?
rhonda Reply
Parts of the body involved in homeostasis
Favour Reply
liver
Kirk
A collection of similar-typed cells working together to perform a specific function is called what?
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tissue
Van
which structure consists of a phospholipid bilayers and it is known as the mosaic model?
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cell membrane
Abanoub
what is diffusion
Patricia Reply
a passive process of transport
Jamar
Random movement of gas molecule from region of high concentration to a low concentration.
praise
10 difference between male and female reproductive system
Ina Reply
if a phase is omitted ,what will happen to the cell
Ewemoje Reply
discribe advantages and disadvantages of asexual and sexual reproduction
basi Reply
Compared to separate sexes and assuming self-fertilizing is not possible, what might be one advantage and one disadvantage to hermaphroditism?
Gift Reply
is nature of their environment among
basi
cool 😎
Precious
why are the laws of thermodynamics considered laws of nature and not scientific theories
Yolonda Reply
archea were given their own separate domain because they are?
Yolonda
which of the following is a basic component of all of the others?
Yolonda
which of the following organization levels is the least inclusive
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which cell feature is absent in bacterial cells
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which metric movies the base unit of measurement by one thousandth (0.001)?
Yolonda
What is biology?
Blessing Reply
List the branches of biology
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List the branches of biology
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List the branches of biology
Blessing
Ecology,Anatomy,genetics,biophysics,biochemistry,microbiology and biotechnology.
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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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